Hello, this could be of help: https://owl.english.purdue.edu/owl/resource/540/01/ If you have other questions, feel free to contact me.

It's not possible to teach you the use of articles in one short answer. But on the issues you mentioned, we say "the earth", as a synonym for "the world", "the globe", because there is only one of them. It would be obvious in context to the reader or listener exactly what you were referring to. On the other hand, each planet has a name, Mars, Venus etc. Our planet is called "Earth" (note the capital E). Names of planets do not add "the". We say "the president of e.g. the Republic of Ireland", because this identifies the unique person we are referring to. The listener then knows exactly who it is. Names of people don't come with "the", and this includes when we give them titles e.g. Mr, Mrs, Professor, Chairman, Prime Minister, President. So the current president of the Republic of Ireland is called "President Higgins".

The, a and an, are determiners, specifically, they are called articles. There are many determiners in English and they function somewhat like adjective in as much as they determine something about the noun they are modifying. All determiners, including the two articles, refer to nouns in either a specific sense or a general sense. 'The' is specific. It is used to refer to specific things we know about. For example, if I say pass me the hammer," I'm talking a about a specific hammer, the one that both I and the person I'm speaking to know about. But if a say "pass me a hammer," I am not asking for a specific hammer, it could bey any hammer. A and an, are non specific, or general. Let's say I have a three chocolate candies, one with a nut, one with a cherry and one is just pure chocolate. I ask you, "Would you like a chocolate?" It could be any one of the three. You look at the chocolates and ask, "can I have the cherry chocolate?" You use 'the' because you are referring to a specific chocolate. A or an are used in the singular only. We do not say, 'can I have a chocolates?' We say, 'can I have a chocolate?' The can be used with both plural and singular nouns. "Can I have all of the chocolates?" We do not use 'the' with proper nouns, titles or the names of places countries or things unless they include 'the' in the title or name. So, it is The United States but not The China. It's just China because that is the full name of the country. Also we to not usually use articles with collective nouns, like chocolate, sunshine, water, pie, and so on. We don't say, 'I need the water,' or 'I need a water.' We just say, 'I need water.'

English has two articles: the and a/an. The is used to refer to specific or particular nouns; a/an is used to modify non-specific or non-particular nouns. We call the the definite article and a/an the indefinite article. For example, if I say, "Let's read the book," I mean a specific book.